Reverting atol default to None in isclose and allclose
Reverting atol default to None in isclose and allclose

This is necessary because atol cannot have units that are incompatible
with the objects that are being compared.

Also I made a cut & paste error (oops!) so I'm putting isclose back in
here.

saimn/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,mhvk/astropy,saimn/astropy,lpsinger/astropy,astropy/astropy,larrybradley/astropy,larrybradley/astropy,pllim/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,astropy/astropy,lpsinger/astropy,StuartLittlefair/astropy,dhomeier/astropy,pllim/astropy,larrybradley/astropy,astropy/astropy,astropy/astropy,StuartLittlefair/astropy,saimn/astropy,pllim/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,larrybradley/astropy,mhvk/astropy,dhomeier/astropy,saimn/astropy,dhomeier/astropy,aleksandr-bakanov/astropy,mhvk/astropy,lpsinger/astropy,pllim/astropy,pllim/astropy,dhomeier/astropy,StuartLittlefair/astropy,saimn/astropy,astropy/astropy,mhvk/astropy,StuartLittlefair/astropy,aleksandr-bakanov/astropy,StuartLittlefair/astropy